Glycogen degradation is precisely controlled by multiple interlocking mechanisms. The focus of this control is the enzyme glycogen phosphorylase. Phosphorylase is regulated by several allosteric effectors that signal the energy state of the cell as well as by reversible phosphorylation, which is responsive to hormones such as insulin, epinephrine, and glucagon. We will examine the differences in the control of two isozymic forms of glycogen phosphorylase: one specific to liver and one specific to skeletal muscle. These differences are due to the fact that the liver maintains glucose homeostasis of the organism as a whole, whereas the muscle uses glucose to produce energy for itself.

The dimeric phosphorylase exists in two interconvertible forms: a usually active phosphorylase a and a usually inactive phosphorylase b (Figure 21.10). Each of these two forms exists in equilibrium between an active relaxed (R) state and a much less active tense (T) state, but the equilibrium for phosphorylase a favors the active R state, whereas the equilibrium for phosphorylase b favors the less-active T state (Figure 21.11). The role of glycogen degradation in the liver is to form glucose for export to other tissues when the blood-glucose level is low. Consequently, we can think of the default state of liver phosphorylase as being the a form: glucose is to be generated unless the enzyme is signaled otherwise. The liver phosphorylase a form thus exhibits the most responsive R ↔ T transition (Figure 21.12). The binding of glucose to the active site shifts the a form from the active R state to the less-active T state. In essence, the enzyme reverts to the low-activity T state only when it detects the presence of sufficient glucose. If glucose is present in the diet, there is no need to degrade glycogen.

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In contrast to the liver isozyme, the default state of muscle phosphorylase is the b form, owing to the fact that, for muscle, phosphorylase must be active primarily during muscle contraction. Muscle phosphorylase b is activated by the presence of high concentrations of AMP, which binds to a nucleotide-binding site and stabilizes the conformation of phosphorylase b in the active R state (Figure 21.13). Thus, when a muscle contracts and ATP is converted into AMP by the sequential action of myosin (Section 9.4) and adenylate kinase (Section 16.2), the phosphorylase is signaled to degrade glycogen. ATP acts as a negative allosteric effector by competing with AMP. Thus, the transition of phosphorylase b between the active R state and the less-active T state is controlled by the energy charge of the muscle cell. Glucose 6-phosphate also binds at the same site as ATP and stabilizes the less-active state of phosphorylase b, an example of feedback inhibition. Under most physiological conditions, phosphorylase b is inactive because of the inhibitory effects of ATP and glucose 6-phosphate. In contrast, phosphorylase a is fully active, regardless of the levels of AMP, ATP, and glucose 6-phosphate. In resting muscle, nearly all the enzyme is in the inactive b form.

Unlike the enzyme in muscle, the liver phosphorylase is insensitive to regulation by AMP because the liver does not undergo the dramatic changes in energy charge seen in a contracting muscle. We see here a clear example of the use of isozymes to establish the tissue-specific biochemical properties of the liver and muscle. In human beings, liver phosphorylase and muscle phosphorylase are approximately 90% identical in amino acid sequence, yet the 10% difference results in subtle but important shifts in the stability of various forms of the enzyme.

Not only do the biochemical needs of liver and muscle differ with respect to glycogen metabolism, but the biochemical needs of different muscle fiber types also vary. Skeletal muscle consists of three different fiber types: type I, or slow-twitch muscle; type IIb (also called type IIx), or fast-twitch fibers; and type IIa fibers that have properties intermediate between the other two fiber types. Type I fibers rely predominantly on cellular respiration to derive energy. These fibers are powered by fatty acid degradation and are rich in mitochondria, the site of fatty acid degradation and the citric acid cycle. As we will see in Chapter 22, fatty acids are an excellent energy storage form, but generating ATP from fatty acids is slower than from glycogen. Glycogen is not an important fuel for type I fibers, and consequently the amount of glycogen phosphorylase is low. Type I fibers power endurance activities. Type IIb fibers use glycogen as their main fuel. Consequently, the glycogen and glycogen phosphorylase are abundant. These fibers are also rich in glycolytic enzymes needed to process glucose quickly in the absence of oxygen and poor in mitochondria. Type IIb fibers power burst activities such as sprinting and weight lifting. No amount of training can interconvert type I fibers and type IIb fibers. However, there is some evidence that type IIa fibers are “trainable”; that is, endurance training enhances the oxidative capacity of type IIa fibers while burst activity training enhances the glycolytic capacity. Table 21.1 shows the biochemical profile of the fiber types.

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In both liver and muscle, phosphorylase b is converted into phosphorylase a by the phosphorylation of a single serine residue (serine 14) in each subunit. This conversion is initiated by hormones. Low blood levels of glucose lead to the secretion of the hormone glucagon. Fear or the excitement of exercise will cause levels of the hormone epinephrine to increase. The rise in glucagon and epinephrine levels results in phosphorylation of the enzyme to the phosphorylase a form in liver and muscle respectively. The regulatory enzyme phosphorylase kinase catalyzes this covalent modification.

Comparison of the structures of phosphorylase a in the R state and phosphorylase b in the T state reveals that subtle structural changes at the subunit interfaces are transmitted to the active sites (Figure 21.10). The transition from the T state (the prevalent state of phosphorylase b) to the R state (the prevalent state of phosphorylase a) entails a 10-degree rotation around the twofold axis of the dimer. Most important, this transition is associated with structural changes in α helices that move a loop out of the active site of each subunit. Thus, the T state is less active because the catalytic site is partly blocked. In the R state, the catalytic site is more accessible and a binding site for orthophosphate is well organized.

Phosphorylase kinase activates phosphorylase b by attaching a phosphoryl group. The subunit composition of phosphorylase kinase in skeletal muscle is (αβγδ)₄, and the mass of this very large protein is 1300 kDa. The enzyme consists of two (αβγδ)₂ lobes that are joined by a β₄ bridge that is the core of the enzyme and serves as a scaffold for the remaining subunits. The γ subunit contains the active site, while all of the remaining subunits (≈90% by mass) play regulatory roles. The δ subunit is the calcium binding protein calmodulin, a calcium sensor that stimulates many enzymes in eukaryotes (Section 14.1). The α and β subunits are targets of protein kinase A. The β subunit is phosphorylated first, followed by the phosphorylation of the α subunit.

Activation of phosphorylase kinase is initiated when Ca²⁺ binds to the δ subunit. This mode of activation of the kinase is especially noteworthy in muscle, where contraction is triggered by the release of Ca²⁺ from the sarcoplasmic reticulum (Figure 21.14). Maximal activation is achieved with the phosphorylation of the β and α subunits of the Ca²⁺-bound kinase. The stimulation of phosphorylase kinase is one step in a signal-transduction cascade initiated by signal molecules such as glucagon and epinephrine.

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